Doesn't this work only for an append-only structure? If I'm updating existing records, and the MySQL master at Site A gets updated, then goes down before Site

Doesn't this work only for an append-only structure? If I'm updating existing records, and the MySQL master at Site A gets updated, then goes down before Site B is updated.. I've got an inconsistent setup. Been thinking about Master-Master MySQL replication recently as we have a system that's duplicated and taken offline each summer (to run a summer camp), and looking for a way to sync changes in it back to the main 'live' MySQL database.